(1) Field of the Invention
The present invention relates to a direct-heated gas-flow measuring apparatus having a film resistor which serves as a temperature detecting means as well as an electric heater. Such a direct-heated gas-flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
(2) Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A gas-flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, which is, however, disadvantageous in scale, response speed characteristics and the like. Recently, airflow meters having temperature-dependent resistors have been developed, which are advantageous in scale, response speed characteristics, and the like.
There are two types of airflow meters having temperature- dependent resistors, i.e., the heater-type and direct-heated type. The heater-type airflow meter may consist of an electric heater resistor provided in an intake-air passage of an engine and two temperature-dependent resistors arranged on the upstream and downstream sides of the electric heater resistor. In this case, the temperature-dependent resistor on the downstream side is used for detecting the temperature of air heated by the heater resistor, while the temperature-dependent resistor on the upstream side is used for detecting the temperature of non-heated air. The current flowing through the heater resistor is controlled for a constant difference in temperature between the two temperature-dependent resistors, thereby detecting the voltage applied to the heater resistor as the mass flow rate of air.
In this heater-type airflow meter, if no temperature- dependent resistor upstream is provided and the current of the heater resistor is controlled for a constant temperature of the downstream temperature-dependent resistor, the voltage applied to the heater resistor is detected as the volume flow rate of air.
On the other hand, the direct-heated type airflow meter may consist of a film resistor which serves not only as an electric heater, but also as a temperature-detecting means for detecting the temperature of the heated air. Also, the direct-heated type airflow meter may consist of a temperature-dependent resistor for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor is controlled for a constant difference in temperature between the film resistor and the temperature dependent resistor, thereby detecting the voltage applied to the film resistor as the mass flow rate of air. In this direct-heated type airflow meter, too, if no temperature-dependent resistor is provided and the current of the heater resistor is controlled for a constant temperature of the film resistor, the voltage applied to the film resistor is detected as the volume flow rate of air.
Since the film resistor of the direct-heated type airflow meter serves as a temperature-detecting means for heated air, that is, no additional temperature detecting means for heated air is necessary, the direct-heated type airflow meter is smaller in size than the heater-type airflow meter.
In the direct-heated type airflow meter, the film resistor may consist of an insulating substrate such as a ceramic substrate or monocrystalline silicon substrate, a resistance pattern of platinum (Pt), gold (Au), etc. on the insulating substrate, and a heat-resistant resin on the resistance pattern. It is impossible to burn off suspended particles (mainly, carbon particles) adhered to the boundary region and the stagnation region of the film resistor, since a temperature higher than 800.degree. C. is required for such burn-off and the heat-resistant resin has a low melting temperature such as 400.degree. C. Therefore, the suspended particles adhered to the film resistor remain, increasing the heat capacity and reducing the heat dissipation characteristics, thereby reducing the sensitivity and response speed characteristics of the airflow meter.